Cobalt-base alloy



2,996,379 COBALT-BASE ALLOY William H. Faulkner, Kokomo, Ind., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed Dec. 4, 1958, Ser. No. 778,052 6 Claims. (Cl. 75-171) This invention relates to a cobalt-base alloy for high temperature service and, more particularly, to a cobaltbase alloy suitable for high temperature service in either cast or wrought form.

Research directed to increasing-the power and efficiency of jet propulsion and gas turbine engines has provided designs for engines that will operate at higher temperatures than currently known metals and alloys can withstand. Metals and alloys capable of withstanding these high temperature stresses are avidly sought therefore. Of especial interest are alloys having high creep resistance strength which are thereby suitable for extended service. Furthermore, alloys are sought which, in addition to possessing outstanding high temperature strength, must also be capable of being fabricated into useful parts by known methods.

In some cases the alloys may be employed in the form of castings, While for other operations these alloys must be capable of being hot-worked into sheet and strip, or subjected to cold-working or machining. Furthermore, materials fabricated from these alloys must have good oxidation, impact and thermal shock resistance.

While several alloys have been proposed for high temperature service under the conditions set forth above, none have possessed the required combination of high strength, high heat resistance and high workability.

The primary object of this invention, therefore, is to provide an alloy which has higher tensile, stressrupture, and creep-rupture strength than alloys heretofore known, as well as increased ductility and impact resistance, to enable its use in a wide variety of applications at temperatures higher than the limiting temperatures of currently known alloys of this type and up to a temperature of about 1800 F.

Another object of this invention is to provide an alloy with high oxidation resistance and fatigue strength at elevated temperatures.

A further object of the invention is to provide an alloy suitable for use at elevated temperatures which may be fabricated by casting or forging, and which can be coldworked to increase its hardness.

Other aims and advantages of the invention will be apparent from the following description and appended claims.

In accordance with the present invention, a cobaltbase alloy is provided having compositions defined by the ranges given in Table A.

To this composition should be added at least one modifying element selected from the group consisting of titanium, magnesium and zirconium such that the mininited States Patent 2 mum amount of said modifying elements present is defined by the following equation:

percent by Weight of the alloy wherein Ti equals the percent by weight titanium, Mg equals the percent by weight magnesium, Zr equals the percent by weight zirconium, the maximum allowable amount of said titanium present being 2 percent by weight, the maximum allowable amount of said magnesium present being 0.50 percent by Weight, and the maximum allowable amount of said zirconium present being 1 percent by Weight.

According to Equation 1 if titanium is the only modifying metal selected, there must be a minimum of 0.05 percent titanium present. Or if magnesium is the only modifying metal selected, there must a minimum of 0.001 percent magnesium. Similarly there must a'minimum amount of 0.05 percent zirconium present, if that metal is the only one selected. Of course more than one modifying metal may be selected and used in the alloy provided there is a minimum amount of said modifying metals present as defined by Equation 1 and up to the maximum amounts of each modifying metal listed above.

Silicon and manganese may be present in amounts up to 2 percent by weight of each.

A preferred composition is defined by the ranges listed in Table B.

Table B Constituents: Percentage by weight Chromium 19 to 21.

Tungsten 11.5 to 13.5. Molybdenum 3 maximum. Iron lmaximum. Carbon 0.43 to 0.53.

Cobalt Balance. Nickel 1 maximum.

"Boron 0.03 to 0.07. Nitrogen 0.04 maximum.

To this composition there should be added at least one modifying metal selected from the group consisting of titanium, magnesium and zirconium such that the minimum amount of said modifying metals present is defined by the following equation:

percent by weight of the alloy wherein Ti equals the percent by weight of titanium, Mg equals the percent by weight of magnesium, Zr equals the percent by weight of zirconium, the maximum allowable amount of said titanium present being 1 percent by Weight, the maximum allowable amount of said magnesium present being 0.50 percent by weight, and the maximum allowable amount of said zirconium present being 1 percent by weight. According to Equation 2 the minimum amounts of titanium, magnesium and zirconium present in the preferred form of the alloy when only one of these modifying metals is selected is as follows: 0.10 percent titanium or 0.001 percent magnesium or 0.10 percent zirconium. When more than one of these metals is used, there should be enough of those selected to represent at least the minimum defined by Equation 2. The maximum of each metal to be used in any case is also listed above.

Silicon and manganese may be present in amounts up to 1 percent by weight of each. a a

By maintaining the composition within the specified critical limits, alloys are produced which exhibit the superior strength, creep-resistanceand workability d'ee scribed herein.

Chromium and tungsten both improve the high temperature strength of the alloy, and give the alloy increased ability to resist deformation while under severe loading. Tungsten may be complemented by molybdenum in amounts up to percent by weight molybdenum. The addition of molybdenum will increase high temperature tensile and stress-rupture strength at the expense of 'a' lo'ss of room temperature ductility. The'refo'r e, th'e molybdenum content islimited in the preferred embodiment to '3 percent. Y

Wh'le the presence of 'nickel increases toughness at lower temperatures it causes a reduction in high temp'erature strength. Therefore the nickel content should be adjusted to achieve specific desired results. For'high temperature service the nickel content should not be "greater than one percent.

Carbon and boron are necessary additions for ductility and strength in products composed of this alloy. Although the addition of boron greatly increases the'streng'th of this alloy, boron is detrimental to oxidation resistance, and therefore the desired level of boron content is dependent upon the anticipated service temperature of the alloy. Howcver, while higher carbon contents are suitable for casting operations, when the alloy is to be rolled both the carbon and boron contents shouldbe lowered.

The high temperature strength of the cobalt matrix'is further increased by the existence of a precipitated phase; composed of carbides of the Cr C type. Of especial importance are the rates of diffusion of chromium and carbon in the cobalt-rich matrix. If these elements are able to diffuse rapidly through the matrix, the carbides are readily formed and grow rapidly yielding a hardening through precipitation. However, beyond a limiting size and concentration, these carbides are no longer useful in increasing the strength of the alloy. Therefore by limiting the dilfusion of carbon and chromium, the rate of growth of these carbides is diminished and their'size'kept within limits with the desired result that the'allo'y achieves maximum strength from this precipitatedphase. v, Tungsten, chromium, and the nickel are efiective in controlling the diifusion of carbon and the formation of carbides.

Furthermore in ordinary chromium-cobalt alloys, a condition occurs wherein alternate layers of the cr c, type carbides and the cobalt-rich matrix form. Since the presence of this larnellar constituent lowers the fatigue resistance of the alloy, efforts were made to prevent its occurrence. This lamellar .constituent. is formed during cooling from the molten statev and is not easilylaltered or removed by heat-treatment. In. the alloy of this invention, however, the lamellar constituent is suppressed by the interactive effects of chromium; carbon, 'ntrogen, and titanium, magnesium or zirconium singly or in combi; nation. The fatigue resistance of products composed of the alloy is thus raised, especially at elevated'tempera tures. The elimination of this lamellar constituent and the.

control of the rate of diffusion of the carbides'are the basis for the chosen composition of the alloy and the ranges given in the preceding paragraphs.

Manganese and silicon may be present in amounts up to 2 percent by weight of each. These elements are found as impurities in the constituents of the alloy and are useful as deoxidizers. However for a preferred'composition of the alloy their'presence should be limited) one percent maximum amounts of each.

Iron may be present inamounts up to 3 percent; but

TABLE C Stress-rupture data at 1700 F. and 20,000 p.s.i.

Nitrogen content, percent by weight Life, Hours Elongation,

' Percent Therefore the nitrogen content should be kept below 0.06 percent 'by 'weighnand preferably below 0.04 percent for maximum strength in the alloy.

On the other "hand, the presence of titanium, magnesium, and zirconium, alone ori'in combination, increases the stress-rupture life of the alloy at elevated temperatures. Furthermore, the addition of these metals also increases ductility. Thebeneficial efiects of titanium; magnesium and zirconium additions are'shown in Tables D, E and F. These tests were performed on cast specimenshaving the following composition by weight: 20 per- 'c'entehro-m' m, 1225 "percent tungsten, 0.48 percent carbbn,f0.05=percent. boron, 0.03 percent nitrogemand the balance"substantiallyJallcobalt with varying additions of titanium andmagnesium.

In Table D, the samples from the a subgroups represent three original heats'of the alloy. The subgroups designated "band c 'demonstrate'theefie'cts of additions offmagnesiu'm; and .magnesium and titanium, re-

spectively, to theoriginal heat. 7

Since thetitam'um and magnesium additions Were-pan tiaiiy va orized out of the melt, the analysis of the titanium 'andinagnesium contents of the casfproductsis also given.

The in'creasefinstress rupture life as 'a' result of theadclitions is gle firgitein each'c'ase. In each case the amount of magnesium in the icast material was below the level fof exact quantitative determination, but it was ascertained spectrographically that the metal was present in amounts -equalfto aboutf0.00l percent by weight. The small am ums r magnesium're'maining inthe alloy prove to be eflectivejhowever. The-presence of titanium nitrides has been 'noted'inthe structures of materials to which 'titaiiium'ha's been added, indicating removal of nitrogen from the matrix.

TABLE D Efiect of tztanzum and magnesium addztzons on: stressrupture life at 1700" F. and 20,000 p.s.z.

, 'Additiontothe Analysis of titanium I molten alloy, and magnesium Life, Elonga- No. percent by weight content of cast alloy, Hours tion, H I V v 7 percent by weight percent 10.... None 9. 8 =5 10 0.10 Magnesium. trace of magnesium 42.1 38 1c 0:10 Magnesiurn+ trace of magnesium, 65. 5 l8 0.10 Titanium. 0.06 titanium.

None .L 23.2 3 0.10 Magnesium. trace of magnesium 81. 9 26 0.10 Magnesium-I- trace of magnesium, 155. 9 33 0.25 Titanium. 0.15 titanium. None r 16. 2 21 0.10 Magnesium... trace of magnesium... -5l.-3 25 0.10 Magneslum+ trace of magnesium, 136. 6 15 1.0 titanium. 0.73 titanium.

Table E,demonstrates the effects of additions of titanium 'tofthe'alloy. The'results of tests conducted on specimens containing varying amounts of titanium all indicate an" increase in stress rupture life with additions of titanium. p

I Table Fsndwsrhe effects of additions of titanium and zirconium to the alloy. Again there is a marked increase in stressrupture life accompanying the addition of these elements,

TABLE B Efiect of titanium additions on stress-rupture life at 1700 F. and 21,000 p.s.i.

Titanium addition, Percent by weight Life, Hours Elongation,

Percent 1 An extrapolated value.

TABLE F Efiect of titanium and zirconium additions on stressrupture life at 1700 F. and 22,000 p.s.i.

Titanium, Percent Zirconium, Percent by weight by weight Life, Elonga- No. 7 Hours tion,

Percent Addition Analysis Addition Analysis 5a none none 2 5 5b 2. 5 1. 45 none 19. 5 45 5c 2.0 1. 87 none 64. l 5d none 0. 50 11. 1 31 5e 1.0 0.77 0. 50 86. 1 16 1 Since the titanium and zirconium additions were partially vaporized out of the melt, the analysis of the titanium and zirconium content of the cast alloy is also given.

2 An extrapolated value.

From the results of Tables D, E and F, the beneficial efiects of additions of titanium, magnesium, and zirconium, either alone or in combination, are evident. Stressrupture life and ductility at elevated temperatures show marked increases as a result of these additions. It is to be noted that titanium, magnesium and zirconium may be replaced by other elements of similar chemical characteristics, such as calcium, barium or vanadium. Magnesium may be replaced in equally effective amounts by calcium or barium; titanium and zirconium may be replaced by ar. equally effective amount of vanadium.

The alloy may be melted by standard furnacing procedures, such as by induction heating, but the melting should be conducted in the protective atmosphere of an inert gas, such as argon. This shielding of the molten .metal by an inert gas will prevent contamination by the nitrogen in the air. The alloy may be used in the as-cast condition. A heat treatment is beneficial in enhancing the properties of the alloy, but it is not required.

Table G shows the short time tensile properties of this alloy in the as-cast condition.

TABLE G Tensile test data for cast material This alloy may also be fabricated by hot or cold working into useful parts. Alloys may be produced that can be mechanically fabricated by known methods to produce such items as wire, strip, rod, plate, bar, sheet, tubings, forgings, and extruded shapes. The preparation of ingots for use in these forging operations should preferably include melting under vacuum conditions to remove dissolved gases.

These products all have excellent mechanical properties at elevated temperatures. The alloy may also be 7 rolled into sheets.

Alloys having especially significant mechanical properties in the wrought form may be prepared without the addition of the modifying metals titanium, magnesium or zirconium. These alloys are disclosed and claimed 75 .6 in my copending application entitled Cobalt-Base Alloys, Serial No. 778,053, filed concurrently herewith, wherein compositions are defined containing from 19 to 22 percent by weight chromium, from 11.5 to 13.5 percent by weight tungsten, carbon below about 0.25 percent by weight, and boron below about 0.15 percent by weight, and the balance substantially all cobalt and incidental impurities. These alloys possess higher mechanical properties in the wrought form than heretofore known alloys.

Table H gives typical stress-rupture data for sheet products having the following composition by weight: 20 percent chromium, 12.35 percent tungsten, 0.16 percent carbon, 0.005 percent boron, 0.48 percent silicon, 0.72 percent manganese, and the balance substantially all cobalt.

TABLE H Stress-rupture data for 0.040 inch thick sheet Stress, Life, Elonga- Temperature, F. p.s.i. Hours tlon,

Percent In the wrought form, the alloy has sufficient ductility to undergo cold-working to increase hardness. Table I shows the results of cold-working on sheet products.

TABLE I Condition: Hardness, Rockwell C test As rolled 31 Cold-reduced 20 percent 49 Cold-reduced 25 percent 51 In addition the cold-worked alloy may be further hardened by heat treatment. Heat treatments at temperatures up to about 12 00 E. will cause an increase in hardness without subjecting the alloy to temperatures so high as to cause distortion in the finished parts.

What is claimed is:

1. A cobalt-base alloy characterized by improved stress-rupture strength at temperatures of about 1700 F., said alloy consisting essentially of between 15 and 25 percent by weight chromium, between 10 and 15 percent by weight tungsten, up to 5 percent by weight molybdenum, up to 3 percent by weight iron, up to about 1 percent by weight nickel, up to 0.70 percent by weight carbon, up to 0.20 percent by weight boron, up to 0.04 percent by weight nitrogen, manganese and silicon in amounts up to 2 percent by weight of each, at least one modifying the metal selected from the group consisting of titanium, magnesium and zirconium, the minimum amount of modifying metal being defined by the following equation Ti+5=0 Mg+Zr=0.05 percent by weight of the alloy wherein Ti equals the percent by weight titanium, Mg equals the percent by weight magnesium, Zr equals the percent by weight zirconium, the maximum allowable amount of said titanium being 2 percent by weight, the maximum allowable amount of said magnesium being 0.50 percent by weight, the maximum allowable amount of said zirconium being 1 percent by weight, and the balance cobalt and incidental impurities, the minimum amount of cobalt being 55 percent by weight.

2. A cobalt-base alloy characterized by improved stressrupture strength at temperatures of about 1700 F., said alloy consisting essentially of between 19 and 21 percent by Weight chromium, between 11.5 and 13.5 percent by weight tungsten, up to 3 percent by weight molybdenum, up to 1 percent by weight iron, up to 1 percent by Weight nickel, between 0.43 and 0.53 percent by weight carbon, between 0.03 and 0.07 by weight boron, up to 0.04 percent by weight nitrogen, manganese and silicon in amounts up to 1 percent by weight of each, at least one modifying metal selected from the group consisting of titanium, magnesium and zirconium, the minimum. amount i hid modifying m'etal being defined by the following 511 on Ti ]-'100 Mg-|-Zr=0.10 percent by weight of the alloy wherein Ti equals theperc ent by weight of titanium, Mg equals the percent by weight of magnesium, Zr equals the percent bysweight of zirconium, the maximum allowable amount of said titanium being 1 percent by weight, the maximum allowable amount of said magnesium being 0.50 percent by weight, the maximum allowable amount of. said Zirconium being 1 percent by weight, and the balance cobalt and incidental impurities. V

3. A cobalt-base alloy characterized by improved stress-rupture strength at temperatures of about 17 00 F., said alloy-consisting essentially of between 19 and 21 percent by weight chromium, between 11.5 and 13.5 percent by weight tungsten, less than 1 percent by weight iron, up to 1 percent by weight nickel, between 0.43 and 0.53 percent by weight, carbon, between 0.03 and 0.07 percent by weight boron, up to 0 .04 percent by weight nitrogen, between about 0.10am 1 percent ,by weight t i tanium,lbetween 0.001 and 0.50 percent by weight magnesiurn, up to l percent by Weight zirconium, and the balance cobalt and incident impurities. 4. A cobaltbase alloy characterized by improved stress-rupture strength at temperatures of about 1700" F., said alloy consisting essentially of about 20 percent by weight chromium, about 12:5 percent by weight tungsten, about 0.48. percent byweight carbon, about 0105 percent by weight boron, about 0.03 percent by weight nitrogen, about 0.15 percent by weight titanium, between 0.001 and 0.10 percent by weightmagnesium, and the balance cobalt and incidental impurities.

5. A cobalt-base alloy characterized by improved stress-rupture strength at temperatures of about 1700" R, said alloy consisting essentially 'of jabout'20 perce'nt by Weight chromium, about 12.5 percent by weight tungsten, about 0.48 percent by weight carbon, about 0.05 percent by weight boron, about 0.03 percent by weight nitrogen, about 1 percent by weight titanium, and the balance cobalt and incidental impurities. H 4

6. A cobalt-base alloy characterized by improved stress-rupture strength at temperatures of about 1700 F., said alloy consisting essentially of about '20 percent by weight chromium,about 12.5 percent by weight tungsten, about 0.48 percent by weight carbon, about 0205 percent by weight-boron, about 0.03.percent by weight nitrogen abont 0,77 per'centby weight titanium, about 0.23 percent by weight zirconium, and the balance 00- balt incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS 2,684,299 Binder July 20, 554 

1. A COBALT-BASE ALLOY CHARACTERIZED BY IMPROVED STRESS-RUPTURE STRENGTH AT TEMPERATURES OF ABOUT 1700*F., SAID ALLOY CONSISTING ESSENTIALLY OF BETWEEN 15 AND 25 PERCENT BY WEIGHT CHROMIUM, BETWEEN 10 AND 15 PERCENT BY WEIGHT TUNGSTEN, UP TO 5 PERCENT BY WEIGHT MOLYBDENUM, UP TO 3 PERCENT BY WEIGHT IRON, UP TO ABOUT 1 PERCENT BY WEIGHT NICKEL, UP TO 0.70 PERCENT BY WEIGHT CARBON, UP TO 0.20 PERCENT BY WEIGHT BORON, UP TO 0.04 PERCENT BY WEIGHT NITROGEN, MANGANESE AND SILICON IN AMOUNTS UP TO 2 PERCENT BY WEIGHT OF EACH, AT LEAST ONE MODIFYING THE METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, MAGNESIUM AND ZIRCONIUM, THE MINIMUM AMOUNT OF MODIFYING METAL BEING DEFINED BY THE FOLLOWING EQUATION 